Along-arc Variations in Subduction Inputs and Mantle Source in Cascadia: Insights from Basaltic Arc Magmas

Abstract:

The Cascade arc spans ~1300 km from northern California into southern British Columbia, and basaltic magmas have erupted throughout the arc. The compositions of arc basalts are particularly useful in discerning mantle origins and inputs to the magmatic system, as basalts have undergone less differentiation en route to the surface. This presentation will draw on both existing datasets and new research to summarize our knowledge of Cascades arc basalt geochemistry and explore along-arc variability in mantle compositions and subduction recycling (oceanic crust and sediment). Cascades basalts are highly variable in composition; at least five types of primitive basalts erupt in the arc, with calc-alkaline basalts (CAB) and low-K tholeiites (LKT, also called high-alumina olivine tholeiites) being the most common. Such variability has been suggested to correlate with mantle heterogeneities and/or mantle melting processes, with CAB originating from fluid-fluxing of the mantle and LKT representing decompression melts (e.g., Leeman et al., 1990; Schmidt et al., 2008). However, recent work has suggested that, at least in some localities, CAB and LKT magmas could originate from a common mantle source (Mullen et al., 2014). A compilation of published primitive (>7 wt% MgO) basaltic magma compositions illustrates potential mantle heterogeneity along the arc, as well as variations in subduction recycling. Increases in melt H₂O contents, radiogenic isotopes, oxygen isotopes, and LILE from north to south along the arc commonly suggest an increase in the amount of subduction component added to the mantle beneath the southern Cascades. The origin of the subduction component (crust vs. sediment) appears variable as well. With recent work on the seafloor sediments offshore of the north Cascades (Carpentier et al., 2010, 2013, 2014), researchers have been able to model the contributions of subducted sediment and crust to the north Cascades arc magmas and have suggested that sediment recycling is minimal (Mullen and Weis, 2013; Mullen and McCallum, 2014). New work on the geochemistry of seafloor sediments offshore of the southern Cascades will complement these existing datasets and provide better constraints on the origin of the increased subduction signature in the southern arc and variations in along-arc subduction recycling.